The present invention relates generally to an overflow-type chemical bath in which an inner bath contains a plurality of holes on its upper sidewall. In this chemical bath, the wet chemical flows over the upper end of each sidewall of the inner bath while passing through the holes at the same time, which reduces the difference of the etching rate between the respective wafers.
In the manufacturing process of semiconductor devices, etching can be performed through two methods: one is wet etching using wet chemical; another is dry etching using gas. Immersion etching and spray etching are included in the former, and plasma etching, ion beam etching and reactive ion etching are included in the latter.
In immersion etching, when the wafers on which insulating layers are deposited are immersed in wet chemical etchant in an inner bath, the insulating layers are etched to the predetermined depth. In this case, the wet chemical etchant is continuously supplied to the inner bath by a circulation system and hence overflows the upper end of each sidewall of the inner bath.
One problem with this method is that etching residue can remain floating around the top surface of the wet chemical etchant. This residue can impair the etching process by reducing the etching speed for the wafer, thus causing different wafers in the same chemical pool to etch at different rates. The overflowing of the wet chemical etchant is meant to flush out this residue, but it is not uniformly successful.
FIG. 1 shows a scheme of a conventional overflow-type chemical bath 10. This chemical bath 10 includes a rectangular-shaped inner bath 11 contained within a rectangular-shaped outer bath 17, the inner bath holding a wet chemical 15. A supply tube 13 is connected to the inner bath, and a discharge tube is connected to the outer bath. A pair of horizontal balance adjusting units 18 are attached to the outside of the outer bath 17. A plurality of robot chucks 23 are provided each for holding one of a plurality of wafers 21 and dipping the wafer 21 into the inner bath for etching. Each of these wafers has a deposition layer that needs to be etched.
The wet chemical 15 is preferably a wet chemical etchant such as hydrogen fluoride (HF), or the like.
With reference to this figure, the wet chemical 15 is supplied into the inner bath 11 through a supply tube 13 connected to the bottom of the inner bath 11. The wet chemical 15 is supplied continuously to the inner bath 11 so that it flows over the upper end of each sidewall of the inner bath 11. The overflowing wet chemical 15 flows into the outer bath 17 and is discharged through the discharge tube 19 connected to the bottom of the outer bath. The discharged wet chemical 15 is then resupplied into the inner bath 11 after passing through a pump and a filter, which are not shown in this figure.
In this conventional chemical bath 10, a pair of horizontal balance adjusting units 18 are provided at either side of the outer bath 17 to horizontally balance the entire mechanism.
The operation of a conventional chemical bath 10 is described below.
The wet chemical 15 is continuously supplied into the rectangular-shaped inner bath 11 of the chemical bath 10 through the supply tube 13 to an uniform amount of about 10 to 12 liters/min. The wet chemical 15 flows over the upper end of each sidewall of the inner bath 11 into the outer bath 17 and is discharged through the discharge tube 19. The discharged wet chemical 15 is then resupplied to the inner bath 11 via the supply tube 13 of after passing through a pump and filter, which are not shown.
As the wet chemical 15 is continually supplied to the inner bath 11, the plurality of robot chucks 23 each hold onto one of the plurality of wafers 21. The robot chucks then lower the wafers 21 into the chemical etchant 15 in the inner bath 11, which causes the deposition layers on the wafers 21 to be etched.
The etching continues for a predetermined time until the layer has been etched to a predetermined depth. Once the predetermined depth is obtained, the robot chucks 23 rise, the wafers 21 emerge from the wet chemical 15, and the etching process stops.
Next, the wafers 21 held by the robot chucks 23 are washed using deionized water in a typical washing process. This completes the etching of the wafers 21.
In this process, however, it is nearly impossible to maintain the horizontal balance of the inner bath 11. As a result, if, for example, the right-positioned sidewall of the inner bath 11 slopes lightly lower than the left-positioned sidewall, the amount of the wet chemical 15 flowing over the upper end of the right sidewall of the inner bath 11 may be larger than that flowing over the upper end of the left sidewall. Likewise, if the left-positioned sidewall slopes lower than the right-positioned sidewall, the wet chemical 15 flowing over the upper end of the left sidewall of the inner bath 11 may be larger than that flowing over the upper end of the right sidewall.
This difference in flow rates over the sidewalls of the inner bath 11 becomes particularly important when you consider the role of the overflow in eliminating etching residue. If one side overflows at a quicker pace than another, wafers 21 closer to that side will have less etching residue interfering with etching and will thus have a greater etch rate. The difference of the etching rate between wafers 21 is undesirable.
Therefore, unless an accurate horizontal balance of the inner bath 11 is maintained, the amount of the overflowing wet chemical 15 may differ at the various sides of the inner bath 11. This makes it difficult to obtain a uniform etching rate for the entire plurality of wafers 21 and diminishes the reliability of the etching process.
Accordingly, in this conventional chemical bath 10, a worker must continually check whether the inner bath is horizontally balanced using a leveling instrument during the etching process. If the chemical bath 10 is not balanced, the worker must adjusting the horizontal balance adjusting units 18, generally using screws or the like, to maintain the horizontal balance of the inner bath 11.